1. Field
This application relates to an electrophotographic photoreceptor and an image forming apparatus including the electrophotographic photoreceptor.
2. Description of Related Art
As electrophotographic photoreceptors, photoreceptors using inorganic materials such as amorphous silicon and organic photoreceptors using organic materials are known.
Organic photoreceptors have excellent optical properties in terms of a width of a light absorption wavelength band and an amount of absorption, and excellent electrical properties such as high sensitivity and stable charging characteristics. In addition, organic photoreceptors have wide material selectivity and may be easily manufactured at a low cost.
Because of these various advantages, organic photoreceptors have recently been widely used instead of inorganic photoreceptors in electrophotographic image forming apparatuses, such as photocopiers, facsimiles, laser printers, and multifunction copiers.
Toner, paper dust, and hydrophilic materials generated by a charging process adhere to a surface of an organic photoreceptor in an electrophotographic image forming apparatus. To remove the toner, paper dust, and hydrophilic materials, cleaning methods involving bringing a urethane-based rubber cleaning blade into contact with a surface of the photoreceptor have been generally used.
However, when a surface of the photoreceptor has a high frictional resistance, a squealing noise may be produced by the cleaning blade, or the cleaning blade may be turned over or inverted. When a blade squealing noise or blade turnover occurs, the blade is gradually damaged, causing toner to leak from the blade. The toner remaining on the surface of the photoreceptor may result in image defects due to poor cleaning performance. Therefore, it is important to suppress an increase of friction of a surface of the photoreceptor over time, to maintain cleaning performance of the surface of the photoreceptor, and to improve abrasion resistance or scratch resistance of the surface of the photoreceptor to facilitate stable acquisition of images for a long time.
In view of printing durability, a protective layer may be formed as a surface layer on a photoreceptor and a curable resin may be introduced into the protective layer to improve mechanical properties of the photoreceptor. For example, JP 3262488 discloses a protective layer including a curable resin and formed on a surface of the photoreceptor.
However, sufficient cleaning performance cannot be obtained merely by increasing mechanical strength. Also, it is difficult to obtain a photoreceptor having a suitable level of all of printing durability, cleaning performance, and scratch resistance.
Thus, attempts have been made to reduce the frictional resistance of the surface of the photoreceptor by improving a slidability thereof. For example, fluororesin particles, such as polytetrafluoroethylene (PTFE) particles, having an excellent lubricating ability may be added to a protective layer on the surface of the photoreceptor to reduce the frictional resistance of the surface. For example, JP 11-202531 discloses a protective layer containing colloidal silica, siloxane resin, conductive particles surface-treated with fluorine-containing compounds, and PTFE particles. JP 2007-86734 discloses a protective layer containing a charge transporting material, an abrasion-resistant resin, and PTFE particles.
JP 2003-140373 discloses a protective layer containing metal oxide particles such as titanium oxide and PTFE particles.
JP 2011-128546 discloses a protective layer containing organic-inorganic composite particles including a fluororesin such as PTFE and an inorganic material.
However, since PTFE molecules included in PTFE particles are not polar, a cohesive force among the PTFE particles becomes excessively large, resulting in extremely poor dispersibility of the PTFE particles in a dispersion liquid used in forming a protective layer. For this reason, a dispersant may be used to improve the dispersibility of PTFE particles in the protective layer. For example, JP 3186010 discloses a protective layer containing PTFE particles and a fluorine-based comb-like graft polymerization resin as a dispersant.
However, when a dispersant is used, a photoreceptor may have deteriorated electrical properties, an increased residual potential, occurrence of image flow, and a deteriorated printing durability, and thus satisfactory image characteristics may not be maintained. That is, when a dispersant having a high electrical resistance is used, a charge in the protective layer is trapped by the dispersant and does not flow adequately, resulting in an increase in a residual potential of the photoreceptor. When an increase in the residual potential of the photoreceptor cannot be suppressed, a sharply outlined electrostatic latent image may not be formed on a surface of the photoreceptor. In addition, in a dispersant having a hydrophilic group that may bind to fluorine atoms of PTFE particles, the hydrophilic group may cause moisture to adhere to a surface of the protective layer. Thus, charge may easily flow, resulting in occurrence of image flow and a low printing durability.
In addition, PTFE molecules included in PTFE particles have a stable molecular structure, and interaction between intermolecular forces is the only constraining force in the protective layer. For this reason, due to scratching by a blade against the surface of the protective layer, PTFE particles may detach from a surface of the protective layer. When PTFE particles detach from the surface of the protective layer, sliding properties (a slidability) of the protective layer may not be maintained, and an increase in the frictional resistance may not be suppressed, resulting in a deterioration of cleaning performance.